Over the last 60 years, the frozen food market has boomed. In 2012, sales were $44 billion (about $140 per person in the US), about 44 times what they were in the mid-1950s. Increased labor force participation by women has increased the opportunity cost of their time. In other words, the cost of preparing meals "from scratch" has risen. A second factor complementing this is a technological improvement, the introduction of the microwave oven. In fact, the widespread acceptance of microwave ovens occurred because of the increasing opportunity cost of time for housework. Entrepreneurs look for areas in which opportunity cost is rising to get some ideas about new technology.

• List two other devices that were developed in response to higher opportunity cost of the time.
• Explain why the devices were developed in response to higher opportunity cost of the time.